SYNOPSIS

DESCRIPTION

sigqueue() sends the signal specified in sig to the process whose PID
is given in pid. The permissions required to send a signal are the
same as for kill(2). As with kill(2), the null signal (0) can be used
to check if a process with a given PID exists.
The value argument is used to specify an accompanying item of data
(either an integer or a pointer value) to be sent with the signal, and
has the following type:
union sigval {
int sival_int;
void *sival_ptr;
};
If the receiving process has installed a handler for this signal using
the SA_SIGINFO flag to sigaction(2), then it can obtain this data via
the si_value field of the siginfo_t structure passed as the second
argument to the handler. Furthermore, the si_code field of that
structure will be set to SI_QUEUE.

RETURNVALUE

On success, sigqueue() returns 0, indicating that the signal was
successfully queued to the receiving process. Otherwise -1 is returned
and errno is set to indicate the error.

ERRORS

EAGAIN The limit of signals which may be queued has been reached. (See
signal(7) for further information.)
EINVALsig was invalid.
EPERM The process does not have permission to send the signal to the
receiving process. For the required permissions, see kill(2).
ESRCH No process has a PID matching pid.

VERSIONS

This system call first appeared in Linux 2.2.

CONFORMINGTO

POSIX.1-2001.

NOTES

If this function results in the sending of a signal to the process that
invoked it, and that signal was not blocked by the calling thread, and
no other threads were willing to handle this signal (either by having
it unblocked, or by waiting for it using sigwait(3)), then at least
some signal must be delivered to this thread before this function
returns.
On Linux, the underlying system call is actually named
rt_sigqueueinfo(), and differs in its third argument, which is the
siginfo_t structure that will be supplied to the receiving process's
signal handler or returned by the receiving process's sigtimedwait(2)
call. Inside the glibc sigqueue() wrapper, this argument, info, is
initialized as follows:
info.si_signo = sig; /* argument supplied to sigqueue() */
info.si_code = SI_QUEUE;
info.si_pid = getpid(); /* Process ID of sender */
info.si_uid = getuid(); /* Real UID of sender */
info.si_value = val; /* argument supplied to sigqueue() */